Abstract

A failed skin bottom of nose belly panel of an aircraft was forwarded to this laboratory for investigation. The failed component was initially investigated by the manufacturer and hence, the component was made available to NAL in the form of a few cut pieces. Examination of the pieces of failed component and the photographs in the report submitted by the manufacturer revealed that material was lost from the skin by fracture resulting in two identical holes, one mirror image of the other. Fractographic study confirmed that the failure was by fatigue mechanism, fatigue cracks being initiated at the fillet of the change of thickness of the skin. After initiation, as the fatigue cracks propagated, the resulting bare metal surface was exposed to the atmosphere which promoted subsequent crack generation by stress corrosion cracking (SCC) mechanism along the fatigue crack propagation path. Analysis suggests that the most probable reason for fatigue crack initiation was due to presence of significant residual stresses in the component/structure. No metallurgical abnormalities in the material of construction were found to be responsible for the failure. A detailed analysis of the failure is presented in this report.